TypeError: unhashable type: 'list'

What you see

visits = {}
name = ["Ada", "Lovelace"]
visits[name] = 1

Traceback (most recent call last):
  File "/home/me/project/main.py", line 3, in <module>
    visits[name] = 1
    ~~~~~~^^^^^^
TypeError: cannot use 'list' as a dict key (unhashable type: 'list')

What it means

A dict finds keys, and a set finds members, by their hash: a number computed from the value. That number must never change while the item is stored. A list can change, so it has no hash, and Python refuses to use it as a key.

Python 3.14 rewrote the message to say what you were doing, "cannot use 'list' as a dict key", with the old wording kept in brackets.

Why it happens

• A list as a dict key, as in the example.
• Adding a list to a set, or set(rows) where each row is a list.
• A dict inside a set or as a key, which gives "unhashable type: 'dict'".
• A tuple that contains a list. Tuples are hashable only when everything in them is; Python 3.14 reports this as "cannot use 'tuple' as a dict key (unhashable type: 'list')".

How to fix it

Use a tuple, which cannot change and so can be hashed:

visits[tuple(name)] = 1          # key is ("Ada", "Lovelace")
unique_rows = {tuple(row) for row in rows}

For a set of sets, use frozenset. For a dict as a key, turn it into something fixed, such as tuple(sorted(d.items())).

Example: a list in a set

seen = set()
seen.add([1, 2])

Traceback (most recent call last):
  File "/home/me/project/main.py", line 2, in <module>
    seen.add([1, 2])
    ~~~~~~~~^^^^^^^^
TypeError: cannot use 'list' as a set element (unhashable type: 'list')

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

How each Python version words it

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

Up to 3.13 both examples print "unhashable type: 'list'". 3.14 and later name the container: "cannot use 'list' as a dict key" or "as a set element", followed by the old text in brackets.